The present invention relates to a process for the preparation of metal sulfide nanoparticles. More particularly, it relates to the said new process employing an efficient, easy and environmentally friendly method for preparing stable, colloidal metal sulfide nano-particles in aqueous solutions using naturally occurring bio-materials such as fungi.
Nano-particles are extremely important materials in different areas ranging from nano-technology, non-linear optics, diode lasers, smart sensors, markers in drugs, gene sequencing to catalysis. Nano-materials can be obtained by various chemical and physical methods. Some examples of physical methods are vapor deposition, lithographic processes and molecular beam epitaxy (MBE) of metal sulphides such as cadmium sulfide, (CdS), lead sulphide (PbS), zinc sulphide (ZnS), silver sulphide (Ag2S), molybdenum sulphide (MoS2) etc. Chemical methods for the preparation of metal sulphide nanoparticles are based on the reaction of metal ions in solution either with H2S gas or Na2S in aqueous medium (V. L. Colvin, A. N. Goldsmith and A. N. Alivisatos, J. Am. Chem Soc. 1992, 114, 5221). In order to stabilize the particles in solution, capping of the particles with thiol derivatives is resorted to. It may be pertinent to mention that yeast has also been used for the preparation of CdS nanoparticles (C. T. Dameron, R. N. Reese, R. K. Mehra, A. R. Kortan, P. J. Carroll, M. L. Steigerwals, L. E. Brus and D. R. Winge, Nature 1989, 338, 596), where the sulphur source is provided by naturally occurring glutathiones present in the yeast. However, this method involves the formation of CdS nanocrystallites inside the cell and extraction of the CdS from the cell is very complex.
The methods mentioned above suffer from drawbacks such as being environmentally hazardous (chemical methods using H2S etc.) and resulting in the quick agglomeration of nano-particles leading to big particles of poor mono dispersity. Although specific capping agents are used in some of the above mentioned methods to restrict the size of the colloidal metal particles and to stabilize the particle size distribution, this makes the whole system multi-step complicated and user unfriendly.
In the view of the above drawbacks of prior art methods, it was necessary to develop a new method overcoming those drawbacks.
The main object of the present invention of provide a new process for the preparation of nano particles of metals naturally occurring fungi under aqueous medium.
Another object is to provide an environmentally friendly process using biological methods avoiding use of hazardous chemicals.
The process used in the present invention reports a new biological method, instead of chemical or physical methods, for preparing colloidal nano-sized metal sulphides. This is for the first time when fungi are used to efficiently prepare colloidal CdS nano-particles from the aqueous solution of cadmium sulphate.
Accordingly, the present invention provides a new process for the preparation of metal sulphide nano particles, which comprises treating wet fungus or fungus extract with a metal sulphate solution at temperature ranging between 15 to 40xc2x0 C. for a period ranging between 2 to 120 hrs., separating the biomass by conventional methods to obtain the corresponding colloidal metal sulphide nano particles.
In one embodiment of the invention, the metal sulphate is selected from the group consisting of cadium sulphate, lead sulphate, zinc sulphate, nickel sulphate, molybdenum sulphate and silver sulphate.
In another embodiment the wet fungus is obtained by growing the Fusarium oxysporum sp. in a conventional culture medium for a period of 2 or more hours at temperature ranging between 15-40xc2x0 C. under aseptic conditions, separating the biomass by conventional methods like centrifugation, washing several times with sterile water, and then incubating the whole reaction mixture at 15 to 40xc2x0 C. and atmospheric pressure.
In another embodiment the fungus extract is obtained by treating the wet fungus with water under aseptic conditions at temperature ranging 15 to 40xc2x0 C. for 2 or more hours.
In yet another embodiment the metal sulphate source is a combination of cadmium ions and a source of sulphur.
In yet another embodiment the source of cadmium ions is a water soluble cadmium salt selected from the group consisting of nitrates, halides and carbonate.
In yet another embodiment, the source of sulphur selected from the group consisting of metal sulphates, sulphites or sulphides exemplified by sodium sulphate, sodium sulphite, sodium sulphide and hydrogen sulphide.
In another embodiment the metal sulphate solution is obtained by dissolving the corresponding metal sulphate in water.
In yet another embodiment of the present invention the concentration of the metal sulphate per gram of the wet fungus or fungus is in the range of 10 to 200 mg. preferably 10-100 mg metal sulphate per gram of the wet fungus extract, and most preferably in the range of 25-100 mg metal sulphate per gram of the wet fungus or fungus extract.
In yet another embodiment the weight ratio of water to the wet fungus or fungus extract is in the range of 1 to 100.
In one embodiment of the invention the fungus is chosen from different species of Fusarium oxysporum, taken as whole cell as wet solid mass or fungus extract.
In another feature of the invention, the reaction of the fungus and a source of metal sulphate in solution is preferably be carried out in water.
In yet another feature of the invention the incubation/reaction temperature is in the range of 15-40xc2x0 C. the preferred range is 23-33xc2x0 C. However, the most preferred range of the incubation/reaction temperature is 25-29xc2x0 C.
The process for the present invention may be described here with examples that are illustrative only and should not be construed to limit the scope of the present invention.